Naturally occurring fibrous biomass produced by plants typically contains a variety of hexose carbohydrates such as glucose, galactose and mannose that are readily fermentable by ethanologenic yeasts to ethanol. Glucose is the sole component of cellulose, but it is also a significant component of hemicellulose, especially in softwoods. Galactose and mannose are the other major hexose carbohydrates that exist in hemicellulose. When Saccharomyces spp. yeasts, typically used in the industrial production of ethanol, are presented with feedstocks comprising mixtures of glucose, galactose and mannose, they will first ferment the glucose and after it is exhausted from the medium, the yeast cells will then adapt to taking up and fermenting the mannose, and then after the mannose is depleted, the yeast will adapt again for metabolizing galactose, which is the most difficult to ferment of the three main carbohydrates derived from hemicelluloses. This type of adaptive metabolic behaviour is called diauxic growth or metabolism (for two carbohydrates) and triauxic growth or metabolism (for three carbohydrates). Between each phase of carbohydrate utilization, there is normally a period of several hours during which time no fermentation occurs while the required transport proteins are induced in the cell membrane of the yeast. This induction phenomenon typically results in significantly extended fermentation times required for complete consumption of mixtures of fermentable carbohydrates derived from hemicellulose. In industrial processes configured for production of ethanol from mixed hexose-carbohydrate feedstock streams produced from lignocellulosic materials such as angiosperm fibers, gymnosperm fibers, and field crop fibers, the fermentation delays caused by enzymatic adjustments during diauxic and triauxic metabolism significantly increase the capital and operating expenditures associated with these processes. Accordingly, strain selection strategies are commonly employed to identify and select yeast stains that are potentially suitable for industrial fermentation processes, based on their efficiencies of converting liquid hexose streams into ethanol in laboratory-scale systems. Suitable exemplary yeast strains for fermenting liquid hexose streams include Saccharomyces cerevisiae T1 for sequentially metabolizing glucose-mannose-galactose, and S. cerevisiae Y-1528 for sequentially metabolizing galactose-glucose-mannose (Keating et. al., 2004, J. Ind. Microbiol. Biotechnol. 31:235-244).
The initial stages of industrial-scale processing of lignocellulosic fibrous materials commonly include physicochemical disruption of the fibers followed by chemical extraction of the disrupted materials using solvents, dilute acid and/or biological conversion of the disrupted materials. The solvent extraction processes typically result in separation of lignins from the oligosaccharide and polysaccharide constituents of the fibers, causing the release of lignins and at least some of the monosaccharides, oligosaccharides and polysaccharides into the extraction solvents. Following the recovery of the solvents, the spent aqueous extraction liquors may then be used as feedstock streams for ethanol production. However, the spent extraction liquors also typically contain significant amounts of lignocellulosic-derived organic compounds such as ketones, aldehydes, carboxylic acids and other such compounds that significantly impair or inhibit microbial fermentative metabolic processes. Such inhibitors are exemplified by furfural, 5-hydroxymethyl furfural, acetic acid and the like. Consequently, selection criteria for identifying commercially useful fermentative microorganisms also include assessments of their tolerance and metabolic performance during extended periods of exposure to inhibitors. Keating et al., 2006, Biotechnol. Bioeng. 93: 1196-1206 have shown that while the fermentation rates of S. cerevisiae strains T1 and Y-1528 declined as the levels of selected inhibitors contained in liquid hexose streams were increased, the overall yields of ethanol in laboratory-scale batch fermentations were not affected.